The Lincoln-Douglas debates of 1858 prominently feature conspiracy arguments—allegations that one’s political opponent is part of a plot to bring about a loathsome result. After contextualizing the debates, the essay examines the major conspiracy arguments, ranging from the charge that Lincoln was plotting to convert both Whig and Democratic parties to abolitionism, to the charge that Douglas was seeking to spread slavery all over the country. The evidence from the debates is drawn upon to consider under what circumstances conspiracy charges become credible and what techniques of argumentation are employed to produce that result. This essay originally appeared in Argumentation and Advocacy, 21 (Fall 1984), 63–75. At the time the journal was called Journal of the American Forensic Association. Frank E. Tutzauer provided valuable research assistance on the original article.